1. Field of the Invention
The invention relates to an exhaust manifold that guides exhaust gas that is discharged from cylinders.
2. Description of the Related Art
As an exhaust manifold of this type, there is known an assembly structure of an exhaust manifold which includes a plurality of branch pipes, and a junction portion in which downstream-side end portions of the branch pipes are converged and housed (e.g., see Japanese Patent Application Publication No. 2005-256785 (JP-A-2005-256785)). In the structure of this junction portion of the exhaust manifold, a reduced-diameter portion that has a reduced diameter is formed in the junction portion, and an oxygen sensor is disposed in the smallest inside diameter portion in the reduced-diameter portion. Therefore, the exhaust gas that flows from all the branch pipes into the junction portion can be gathered in the reduced-diameter portion so as to hit the oxygen sensor. Hence, the oxygen concentration can be highly accurately measured.
In the foregoing structure of the junction portion of the exhaust manifold, the exhaust gas discharged from the branch pipes is guided along wall surfaces of the junction portion so as to flow into the reduced-diameter portion. Therefore, while the exhaust gas that flows into the reduced-diameter portion can be gathered in the reduced-diameter portion and can be caused to hit the oxygen sensor, the exhaust gas passes through the reduced-diameter portion without being sufficiently diffused. Therefore, the oxygen sensor may receive a relatively large amount of exhaust gas discharged from a nearby branch pipe, in a unit time.